We did a little bit of gentle joint busting at Skelf, the Scottish framers gathering. Four scarfs were bought to the event to be tested in the Carpenters Fellowship joint busting rig which records the loading and the deflection in the piece (it's the square steel frame in the background of the second picture).

We also tested a solid piece of wood as a bench mark although the only piece we could use was actually an inch thinner than the jointed pieces so it was not exactly rigorously scientific! The amount of beer everyone had drunk by this time probably put paid to scientific rigor anyway.

As the load is progressively applied the readings from the load cell and the deflection meter get written on the piece with a black marker. The first number you see is the amount of deflection in centimetres and the second is the load in tens of kilos.

A scarf joint is used to join pieces end to end and can be used in various applications in a building, each of which has to resist different amounts of the various forces, tension, compression, shear, bending and twisting. We had decided to test these ones in bending because it usually makes for a quite dramatic show and we were doing this primarily for fun.

Nevertheless the results were very interesting.

I had brought a scissor scarf, which is not a common type, because I had used one recently in a cruck frame at Orwell Farm when we'd been faced with a timber sourcing problem and had to use two pieces to make each of the crucks. With it being in the middle of a curved timber this scarf was potentially subject to some bending stress. The actual scarf I tested was keyed (that's the totally crushed square piece going through the middle of the joint) as well as pegged, in order to mimic the compression which it would be under in the cruck.

Amazingly this joint resisted 10 tonnes of load which is almost twice what is typical for a scarf of this size! It has to be said that it deflected very easily right from the beginning so it was less rigid than the others in our sample, but it was still holding up to the force of the jack at 9cm deflection after which it finally shattered.

After taking it out of the testing rig it sprang back quite a bit so the photos don't show the true amount of deflection it reached. They do show the mess it got into though!